Conventional color laser printers generally include a drum unit that is detachably mounted in a main casing of the printer. The drum unit has a plurality of image forming units provided with respective photosensitive drums on which electrostatic latent images are formed.
A plurality of receiving electrodes for receiving electric power for each of the image forming units is arranged, on a wall of the drum unit, in a direction parallel to a drum-mounting direction in which the drum unit is inserted into the main casing. Also, a plurality of supplying electrodes for supplying electric power to the respective receiving electrodes is arranged on the main casing at appropriate positions such that the supplying electrodes are brought into contact with the corresponding receiving electrodes when the drum unit is mounted in the main casing (see U.S. Pat. No. 6,944,415, for example).
In this type of color laser printer, the receiving electrodes gride or slide over the corresponding supplying electrodes each time the drum unit is inserted into or removed from the main casing. As a result, these electrodes are worn with time.
Particularly, the receiving electrode disposed on the downstream side in the drum-mounting direction and the supplying electrode disposed on the upstream side in the drum-mounting direction are also grided by electrodes other than those corresponding thereto, during the inserting or detaching operation of the drum unit. Thus, those electrodes are worn to a large extent to give rise to a problem of contact failures, and enough electric power may not be supplied to the image forming units.